Kathy Bond-Borie: Are your hydrangeas in the pink?

Guest columnist

The presence or absence of aluminum determines if your hydrangeas are pink or blue.

The beautiful flower heads of hydrangeas announce the summer season just like strawberries and lemonade. Every year there are tempting new varieties of this well-loved plant to welcome to our gardens, and for many gardeners it’s hard to resist planting just one more. You can plant bigleaf hydrangeas (H. macrophylla) in different parts of the garden and enjoy pink flowers on some of the plants and blue on others.

The blooms of the common bigleaf hydrangea change in color according to how acid or alkaline your garden soil is. Acid soils produce blue blooms, whereas less acidic soils assure the gardener of pink flowers. Sometimes you will even see varying shades of pink and blue on the same plant or within the same planting, due to variable pH levels in the soil. The mechanism of color variation in hydrangeas is the presence or absence of aluminum in the blooms. When aluminum is present, the blooms tend to be blue.

When it’s absent, they tend toward pink. Soil pH affects bloom color by making soil aluminum more (low pH or acidic soil) or less (higher pH or basic soil) available for uptake by the plant.

To change the bloom color from pink to blue, add 1/2 cup of sulfur per 10 square feet, mix it into the soil surface, and water it in well. To change the blooms from blue to pink, instead of sulfur mix in 1 cup of lime per 10 square feet.

This procedure doesn’t have overnight results, so be patient. It can take up to a year to change the soil pH and thus the color of the flowers.

For a quicker response in changing flowers from pink to blue, dissolve 1 tablespoon of alum (aluminum sulfate) in a gallon of water and drench the soil around the plants thoroughly three times, four weeks apart, in spring.

To quickly change from blue to pink, substitute 1 tablespoon of hydrated lime for the alum. Always apply these solutions to the soil, not to the blooms or foliage.

Keep in mind that city water supplies are often alkaline, so frequent watering will naturally turn the soil more alkaline, and the blooms brighter shades of pink.

A former floral designer and interior plantscaper, Kathy Bond-Borie has spent 20 years as a garden writer/editor, including her current role as Horticultural Editor for the National Gardening Association.